Method for effecting endothermic dehydrogenation reactions



Jan. 10, 1956 a. E. LIEDHOLM METHOD FOR EFFECTING ENDOTHERMICDEHYDROGENATION REACTIONS Filed March 13, 1955 Fri DRYI N6 H15 REMOVAL aE 2 m g a EU w m PM Rm WW m m m D HEATING FURNACE PRODUCT GAS INVENTOKGEOK LI DHOLM BY Z HIS A RN Y 1 United States Patent METHOD FUREFFECTENG ENDOTHERMIQ DEHYDRUGENATION REACTIGNS 7 George E. Liedholrn,Berkeley, Calif., assiignor to Shell Development Company, Emeryville,Calif a corporation of Delaware "Application March 13, 1953, Serial No.342,139 11 Claims. Cl. 260-668 a This invention relates to an improvedmethod for carrying out'endothermic dehydrogenation of variousdehydrogenatable materials in the presence of added hydrogen. It relatesmore particularly to a particular method of contacting the reactants,hydrogen, and catalyst in commercial' practice.

Hydrogenation and dehydrogenation are the reversed directions of asingle reaction. The concentrations of a dehydrogenated compound and itsundehydrogenated parent in equilibrium in any case is dependent upon thepre- 7 vailing conditions and may be calculated from known thermodynamicproperties of the compounds in question.

Dehydrogenation is favored at higher temperatures and hydrogenation atlower temperatures. The reaction rate is, of course, favored by highertemperatures.

' It is well known that various dehydrogenatable compounds may bedehydrogenated by bringing them into contact under suitable conditionswith any one of a number of known hydrogenation-dehydrogenationcatalysts. In order to obtain a suitably fast reaction rate and at-thesame time. to allow a favorable equilibrium, it isdesired to employ atemperature near the maximum possible temperature. The maximum possibletempera- I hire is determined in any caseby the onset of excessive-s'ide reactions such in particular as cracking. When operating nearthis maximum applicable temperature the catalyst becomes fouled withcarbonaceous deposits and horny short periods of operation can beapplied. In some 1' cases the catalyst can be regenerated by burning offthe ca'rbonaceous deposits but the necessity of such regenerationmakessuch regenerative processes relatively costly and inefficient.

lowering the maximum possibleconversion by shifting the flrnentionedequilibrium in the-unfavorable direction. This disadvantagectnbecompensated for by some further increase in the reaction temperature,but the improvement obtained by increasing the partial pressure ofhydrogen and'the temperature is limited :by the onset of destructivehydrogenation. Destructive dehydrogenation is to be avoided becauseitleads to the formation of large amounts of gas other than hydrogen whichusually ac- 'cumulates in the recycled product gas thereby decreasingthe protective eifect of the gas, Also, the destructive hydrogenationreaction is quite exothermic and, consequently, the initiation ofdestructive hydrogenation results in a temperature increase whichfurther fosters destructive H hydrogenation and cracking. As a result ofthese factors, any appreciableamount of destrictive hydrogenation, if

allowed to take place, is prone to result in deactivation ofthe'catalyst'and increase in cracking until destruction 'of thefeedis'the predominant reaction.

r 2,730,556 Patented Jan. 10, 1956 Dehydrogenation is endothermic;therefore, when appreciable extents of conversion are desired anappreciable amount of heat must be supplied to the reaction zone. Incommercial application this presents a serious problem. Transfer of thedesired heat to the catalyst bed through the reactor walls is costly,difiicult, and not very satisfactory. It is more practical to supply theheat with the feed to be dehydrogenated and/ or the recycled hydrogengas. This requires, of course, that these materials be supplied at atemperature above the desired reaction temperature. However, the maximumtemperature at which the reactant stream may be heated is strictly andseriously limited firstly, by the tendency for the feed to crackthermally during the preheating step, and secondly, by the necessity ofavoiding localized temperatures sufficiently high to initiate thementioned exothermic destructive hydrogenation in the presence of thecatalyst. Also, even when preheating the feed stream to'the maximumpermissible temperature, there is a very substantial temperature drop inthe direction of the path of travel of the reactant through the catalystbed. Thus, in a typical case, the temperature at the forepart of thecatalyst bed is dangerously close to that causing thermal cracking and/or destructive hydrogenation but the temperature near the exit end ofthe catalyst bed has dropped so far that the conversion is slow and islimited by a less favorable equilibrium. This undesired condition can beimproved by eifecting the conversion in a series of small steps whilereheating the reactant mixture between the steps, but this requires aseries of reactors with interstage heaters and at best gives a sawtoothed temperature profile.

An object of my invention is to provide a method of operation andapparatus therefore which allows catalytic vapor phase dehydrogenationto be carried out more effectively in a more selective and practicalmanner. Generally speaking, this object is attained by the method ofoperation about to be described in which all, or substantia'lly all ofthe heat of the endothermic dehydrogenation reaction is supplied with aportion of recycled and highly heated product gas consisting essentiallyof hydrogen. Sensible heat is thereupon transferred from the hydrogen toa solid finely divided dehydrogenation catalyst which acts as a heatcarrier and is transported in the system by a separate portion ofhydrogen gas. In this system the material to be dehydrogenated is passedin vapor form diluted with hydrogen, up through a fluidized downwardmoving bed of the preheated catalyst. The catalyst, cooled by thereaction continues to pass downward countercurrent to preheatedhydrogen. By this arrangement, reactant vapors are removed from therecirculated catalyst before the same is fully heated thereby avoidingthe severe coking that would otherwise occur at the high temperaturesapplied. A further and important characteristic of the process is thatthe catalyst is saturated with hydrogen before it contacts the materialsto be dehydrogenated. This helps to prevent localized contact of thecatalyst with the material to be dehydrogenated in the absence ofsufiicient hydrogen to protect the catalyst.

The process will be more fully explained in connection with thefollowing description of an example, namely the dehydrogenation of anaphthenic straight-run gasoline to produce aromatics. In thisdescription reference will be had to the flow diagram given in theaccompanying drawing. Referring to drawing, a straight-run gasoiinefaction boiling between about 113 and 228 F. is introduced by line 1 atthe operating pressure, preferably after having been partially preheatedby heat exchange with one of the product streams (not illustrated). Theoperating pressure may be from about p. s. i. g. up to about 1000 p. s.i. g. In the example in question, the operating pressure is 200 p. s. i.g. The feed is preheated in preheating furnace 2 up to approximately thedesired reaction temperature or as near thereto as possible withoutcausing decomposition in the furnace coils. In the case of theparticular feed in question the preheat temperatures are between about920 F. and 975 F. A small amount of recycled gas consisting mainly ofhydrogen is preferably added by line 3 and the mixture is passed to thereactor 4. The amount of hydrogen introduced by line 3 may be quite low,for instance, 0.1 to 1 mole/mole of feed. This amount is not sufiicientin itself to protect the catalyst under the temperature conditionsprevailing and merely supplements the main ficw of hydrogen. It is usedto prevent localized high concentrations of hydrocarbon in the catalystbed near the point of feed injection.

The reactor is essentially an elongated vertically disposed cylindricalvessel provided with means for transporting catalyst from the bottom tothe top. The vessel may be of uniform or non-uniform diameter throughoutits length, e. g., the upper part may be of larger diameter than thebottom part. In the case illustrated, transportation of the catalyst iseffected by withdrawing catalyst through line 5 and valve 6 andtransporting it to the top of the reactor by line 7 by means of a streamof hot hydrogen gas introduced by line 8. Line 7 can it" desired beplaced within the reactor shell. The hydrogen (recycled gas) used forthis purpose is again only a minor amount of the total hydrogen used, c.g., l-Z moles/mole of hydrocarbon feed, and is heated to a hightemperature, e. g., l1501450 F. Line 7 discharges in the upper sectionof the reactor, preferably in the socalled disengaging space above thefluidized bed of catalyst. A cylindrical bafile 9 which is open at bothends causes the incoming mixture of catalyst and gas to swirl in theannular space between the cylindrical baffle and the vessel wall therebydropping out most of the catalyst which falls to the fluidized bed It Acyclone-type separator 11 is provided in the disengaging space of thereactor to effect a more thorough separation of catalyst particles fromthe vapors leaving the reactor. The loading of the cyclone separator 11(which may have one or more stages) is materially decreased by therelatively open grid 12 which is in the disengaging space above thelevel of the fluidized bed of catalyst.

The amount of catalyst transported to the top of the reactor describedmay vary from about 2 to about parts by weight per part of reactantfeed. The catalyst thus transported gradually flows downwardly throughthe vessel to the bottom and is then recycled. The reactor is preferablyprovided with grid plates 13-16. These grid plates have a fairly largeopen area to allow the catalyst to sift downwardly countercurrent to theuprising vapors. in the absence of these grid plates the composition ofthe reacting vapors and the temperature throughout the catalyst bedwould be essentially uniform. This is the result of undesirable backmixing and is a characteristic property of unbafl'led fluidized beds. Te grid plates substantially decrease this mixing; consequently, thedehydrogenated product is not retained in the system for an inordinatelength of time, unreacted material is not passed out of the reactorwithout suificient contact, and there is a generalty increasingtemperature gradient from the level of introduction of the feed up tothe top. More or less uniform conditions prevail, however, in theindividual spaces between the plates. it will be understood that whilefour such grid plates or trays are used in this example, either agreater number or a lesser number may be used. The grid plate 15,situated just below the feed inlet, is relatively important for thereasons which will be later pointed out. The grid plates illustratedconsist of concave plates provided with suitable holes or slots. Theyare placed in depending position, i. e., with the convex side downward.This is to counteract the normal tendency in fluidized beds for theuprising vapor to pass mainly up through the center of the supportingplates. In the arrangement illustrated the pressure required to pass thegas up through the plates and their superimposed beds is somewhatgreater near the center than near the periphery. Other arrangements andconstruction of the plates are possible.

The main stream of the hydrogen gas is preheated to a temperature muchabove that applicable in the reaction zone, e. g., ll50l450 F., inheating furnace 17, and is passed via line 18 into the reactor near thebottom. This hydrogen transfers almost all of its sensible heat to thedescending catalyst and at the same time removes reactant vapors fromthe catalyst and is itself cooled. This is most important since, if thecatalyst is not substantially free of reactant vapors, it becomesseverely coked upon raising its temperatures to the levels in question.Also this hydrogen is the main source of the considerable amount ofhydrogen required in the reaction zone when operating at the relativelyhigh temperatures used. The amount of hydrogen supplied by line 18 mayvary somewhat depending upon the particular feed stock and thetemperature and pressure conditions but is usually between about 1 and10 moles per mole of reactant feed. The total amount of hydrogen heatedin heating furnace 17, on the other hand, may be between 2 and 12 molesper mole of reactant feed.

The effluent mixture of product vapors and hydrogen leaving the reactorat the top by line 19 is at essentially the highest reactiontemperature, e. g., 9l0990 F. This hot mixture carries in suspension asmall amount of catalyst dust which escapes separation in the cycloneseparator 11. This catalyst may be recovered in various ways to avoidfouling of the product recovery equipment. In the case illustrated asmall part of the product is condensed upon passing through a partialcondenser 21. The small amount of condensate containing the small amountof catalyst is collected in the separating tank or knock-out drum 22.The product vapors and hydrogen are then passed through a condenser 23to a high pressure separator 24. The liquid product is passed throughthe low pressure separator 25 and is then withdrawn by line 26 for suchfurther handling as may be desired. The low pressure vent gases consistlargely of hydrogen released from solution in the liquid product upondecreasing the pressure.

The uncondensed material in the high pressure separator consistsessentially of hydrogen but contains small amounts of hydrocarbonvapors, diluent gases, and traces of hydrogen sulfide. This gas ispassed by line 27 to a conventional unit 28 for removing the hydrogensulfide, and it may be passed to unit 29 for the removal of moisture andpreferably also some of the hydrocarbon constituents. The excess, clean,dry gas is removed from the system by line 30 and the major portion isheated in heater 17 up to a temperature between about 1150" and 1450' F.to supply the heat of the dehydrogenation reaction. As previouslypointed out, the main part of this hydrogen stream is passed into thebottom of the reactor where it transfers its heat to the catalyst andthen passes upward to dilute the reactant vapors and protect thecatalyst in the upper section of the reactor. A minor amount of theheated, recycled gas is passed by line 8 to transfer the heated catalystto the top of the reactor, and a minor amount of the recycled gas may beadvantageously mixed with the feed.

It will be noted that, in the case just described, the temperature ofthe catalyst transported to the top of the reactor is above thetemperature of the feed introduced near the middle of the reactor. Also,the temperature of the catalyst at the bottom of the reactor approachesthe high temperature of the preheated recycled gas. There is, therefore,a very desirable temperature gradient set up in the reactor in which thehighest temperature is at the bottom. This temperature is above thatapplicable in the presence of the reactant feed. The temperature at thetop may be somewhat lower but is substantially the highest gen mixture.This not only allows a fast reaction rate .to

ry h n rsi n e rer complet b a presents the most'faverable equilibriumfrom the standpoint of the dehydrogenation reaction. The temperaturenear the middle of the reactor justahove the points where the feedisinjected is the lowest. Herea particularly favorable equilibrium isnot necessary since at the start of the reaction the reaction raterather than the equilibrium is limiting. As the dehydrogenation takesplace during the upward passage of the reactant the reacting vaporsbecome more and more refractory and are .thus capable ofwithstanding'high'er temperatures without cracking and de- 7 activationof'the catalyst. While the described method can be -advantage ouslyapplied at lower or moderate temperature levels, it has the importantadvantage of allowing the dehydrogenation to be carried out at higheraverage temperatures than applicable for the feed in question when usingthe hitherto employedmethods of operation.

The methodof the invention is applicable with the various'kn ownhydrogenation-dehydrogenation catalysts. It

is particularly advantageous with the super-active hydrogenation-dehydrogena'tion catalysts such as those containing metallicnickel, platinum, or palladium since with these catalysts theprotectiveeffect of the hydrogen is especially important and,.on the other band,due to extreme rapidity of the dehydrogenation reaction, the'maintenance'of a temperature gradient such as described is practicallyimpossible by conventional methods. In the case of the relatively'rugged but less active catalysts such as iron oxide, molybdenum sulfide,nickel sulfide, tungsten sulfide,and

chromium oxide, the same problem exists but it is not so 3 serious.

The process of the invention is applicable and can be substituted forthe conventional; processes for endothermic dehydrogenation 'of variousdehydrogenatable organic compounds which can be vaporized. When usingthe mentioned shper-active metal catalysts, the reactant material ispreferably free of compounds containing oxygen, nitrogen,sul fur, andhalogen but they may contain boron,

phosphorous, or silicon. The process is particularly suitable for thedehydrogenation of naphthenic hydrocarbons,

. either individually or in kariousadmixtures, to their correspondingaromatic hydrocarbons. For this operation one of the super-activecatalysts is preferred and the naphthenic hydrocarbon or.naphthenichydrocarbon fraction is-"-preferabl-y substantially free ofsulfur.

/ The use of the relatively open grid plate or tray in the disengagingspace above the fluidized bed in the reactor to decrease the loading onthe cyclone separator is an improvement discovered by a coworker andconstitutes no part of my invention. I

' I claim as my invention:

1. In a catalytic endothermic vapor phase dehydrogenation process theimproved method of contacting the catalyst with the reactant to bedehydrogenated which comprises passing a powdered dehydrogenationcatalyst in the form of a-plurality of semi-isolated fluidized bedsdownward through an elongated reaction vessel, introducing'preheatedreactant vapors into the reaction vessel near the mid-height thereof,withdrawing reacted vapors in admixture with recycle gas consistingmainly of hydrogen from the top of said reaction vessel, separating fromsaid withdrawn mixture a recycle gas consisting largely of hydrogen,heating said recycle gas to a temperature higher than the averagereaction temperature, introducing the larger part of the heated recyclegas into the said reaction vessel near the bottom thereof below thepoint of intro duction of said feed whereby the catalyst in the lowerzones of said vessel is preheated and stripped of reactant vapors andthe thereby cooled recycled gas passes upward and mixes with the feed inthe upper zones of the said vessel, and utilizing a separate minorportion of heated recycle gas to separately transport the heatedcatalyst from near the bottom of said reaction vessel to near the top ofthat the catalyst is a powder containing a metal selected fromthe groupconsisting of nickel, platinum, and palladiufn as the active catalystpromoter.

3. Process according to claim 1 further characterized in that thecatalyst is largely present in a series of semiisolated butinter-communicating superimposed zones, all of which are traversed byrecycled products gas seriatim, whereas only the upper of which aretraversed by the said reactant vapors.

4. Process according to claim 1 further characterized in that a minorportion of said recycled gas is introduced into the reaction zone inadmixture with the feed vapors.

5. Process according to claim 1 further characterized in that thetemperature of the reactant vapors introduced a into the reaction zoneis essentially the maximum temperature at which the reactant may beheated without thermal cracking and the temperature of the catalystabove and below the point of injection of the feed exceeds thistemperature.

6. Process according to claim 1 further characterized in that theheatrequired for the endothermic dehydrogenation is supplied as sensibleheat in recycled product gas and said sensible heat is largelytransferred to the catalyst prior to commingling the main portion ofsaid recycled gas with the reactant vapors.

7. A process for effecting dehydrogenation of a hydrocarbon with afinely divided dehydrogenation catalyst in the presence of a recycledhydrogen which com prises in combination the steps of separating fromthe product stream of such process a product gas consisting essentiallyof hydrogen, dividing said product gas into four separate streams,withdrawing one of said streams as product gas, utilizing the second ofsaid streams as a carrier gas to transport powdered dehydrogenationcatalyst through a confined passage from the bottom to the top of anelongated vertically disposed contact Zone, mixing the third portionwith hydrocarbon to be dehydrogenated and injecting the mixture near themiddle of said elongated contact zone, and passing the fourth portion upthrough the entire length of said elongated contact zone therebystripping occluded hydrocarbon from the catalyst in the lower portion ofsaid elongated contact zone and to serve a diluent for the hydrocarbonintroduced as aforesaid near the middle of said elongated contact zone,and withdrawing product vapors consisting of reacted hydrocarbons andthe combined four streams from the top of said elongated contact zone asthe aforesaid product stream.

8. The method of effecting an endothermic dehydrogenation of ahydrocarbon with a finely divided dehydrogenation catalyst in thepresence of hydrogen which comprises in combination the steps ofproviding a vertically disposed elongated contact zone substantiallyfilled with downwardly flowing fluidized dehydrogenation catalyst,dividing said elongated contact zone into a plurality of semi-isolatedsmaller zones by means of barriers restricting free intermixing offinely divided catalyst between said zones while allowing downward flowof said catalyst serially through said zones, maintaining thetemperature in the upper end of said contact zone at an elevatedtemperature above a given average dehydrogenation temperature bycontinuously introducing into said zone preheated dehydrogenationcatalyst suspended in preheated product gas consisting essentially ofhydrogen, maintaining the temperature at an intermediate point in saidelongated contact zone at an elevated temperature below said givenaverage dehydrogenation tempera ture by introducing into said zone atsaid point vapors of the hydrocarbon to be dehydrogenated preheated to atemperature which is below said given average dehydrogenationtemperature and hydrogen separately cooled to a temperature which isbelow' said given averagedehydrogenation temperature, maintaining thetemperature at the bottom of said elongated contact zone at atemperature above said given average dehydrogenation temperature by theintroduction of recycled product gas consisting essentially of hydrogenand preheated to a temperature above said given average dehydrogenationtemperature, passing said latter gas in the absence of hydrocarbonvapors other than those stripped from the descending catalyst up throughthe lower part of said elongated contact zone to the said point ofintroduction of said hydrocarbon vapors, and passing said gas inadmixture Withsaid hydrocarbon vapors through the upper portion of saidelongated contact zone countercurrent to the descending stream ofpreheated catalyst therein.

9. Process for the endothermic dehydrogenation of a hydrocarbonaccording to claim 8, further characterized in that the product gasconsisting essentially of hydrogen is split into four streams which areintroduced into said elongated contact zone as follows: a minor part ismixed with the hydrocarbon prior to introduction into said zone, a minorpart is used to transport preheated and stripped catalyst from thebottom of said zone to the top thereof, a minor part is removed from thesystem as product gas, and the major part is introduced into said zonenear the bottom thereof.

10. The method of effecting an endothermic dehydrogenation of ahydrocarbon with a powdered dehydrogenation catalyst which comprises incombination the steps of preheating hydrogen to a temperature above adesired average dehydrogenation temperature, passing said preheatedhydrogen up through a bed of dehydrogenation catalyst cooled to belowthe said average dehydrogenation temperature, thereby to strip saidcatalyst of occluded hydrocarbons and to transfer heat to said catalyst,preheating hydrocarbon vapors to be dehydrogenated to a temperaturebelow the said average dehydrogenation temperature, commingling thepreheated hydrocarbon with partially cooled hydrogen passed up throughsaid bed as aforesaid and passing the resulting mixture upwardcountercurrent to preheated dehydrogenation catalyst thereby to increasethe temperature of the mixture up to above the said averagedehydrogenation temperature and to dehydrogenate said hydrocarbon,suspending dehydrogenation catalyst preheated to a temperature above thesaid average dehydrogenation temperature by the aforesaid passagetherethrough of preheated hydrogen in a separate portion of hydrogen andcombining the resulting suspension with the mixture of hydrogen andhydrocarbon vapors passed upwardly countea-current to preheateddehydrogenation catalyst as aforesaid, separating the vapors from thesuspended catalyst in the last resulting mixture, withdrawing thehydrocarbon vapors together with the hydrogen used for stripping and theseparate portion of hydrogen as a product vapor stream, and passing theseparated catalyst at a temperature above the said averagedehydrogenation temperature countercurrent to the first aforesaidmixture to heat the same and effect dehydrogenation of the hy drocarbontherein under conditions of an increasing temperature gradient.

11. In a process for efiecting dehydrogenation of a hydrocarbon with afinely divided dehydrogenation catalyst in the presence of recycledhydrogen, the improved method of operating such process which involvesin combination the steps of introducing a stream of pre heateddehydrogenation catalyst in hydrogen at the top of a descending streamof fluidized dehydrogenation catalyst, passing preheated vapor of thehydrocarbon to be dehydrogenated countercurrently to said descendingstream of the powdered dehydrogenation catalyst preheated to atemperature above the temperature of said preheated hydrocarbon vaporsso that there is an in creasing temperature gradient in the hydrocarbonvapors along the path of contact with said catalyst and a decreasingtemperature gradient in the stream of powdered catalyst as it descendsin contact with said hydrocarbon vapors, stripping the dehydrogenationcatalyst of occluded hydrocarbons after said countercurrent contact withsaid hydrocarbon vapors by passing up through said catalyst in a bed astream of hydrogen preheated to a temperature above the temperature ofthe said preheated hydrocarbon vapors whereby said hydrogen is partiallycooled by said catalyst, passing the partially cooled hydrogencontaining stripped hydrocarbons from said bed with said preheatedhydrocarbon vapors countercurrently to said descending stream ofpowdered dehydrogenation catalyst thereby to dilute the said hydrocarbonvapors and be reheated, and passing the stripped catalyst in a confinedstream in hydrogen up to the top of said descending stream.

References Cited in the file of this patent UNITED STATES PATENTS2,602,771 Munday et al. July 8, 1952 2,643,214 Hartwig June 23, 19532,656,304 MacPherson et a1 Oct. 20, 1953

1. IN A CATALYTIC ENDOTHERMIC VAPOR PHASE DEHYDROGENERATION PROCESS THEIMPROVED METHOD OF CONTACTING THE CATALYST WITH THE REACTANT TO BEDEHYDROGENATED WHICH COMPRISES PASSING A POWDERED DEHYDROGENATIONCATALYST IN THE FORM OF A PLURALITY OF SEMI-ISOLATED FLUIDIZED BEDSDOWNWARD THROUGH AN ELONGATED REACTION VESSEL, INTORUCING PREHEATEDREACTANT VAPORS INTO THE REACTION VESSEL NEAR THE MID-HEIGHT THEREOF,WITHDRAWING REACTED VAPORS IN ADMIXTURE WITH RECYCLE GAS CONSISTINGMAINLY OF HYDROGEN FROM THE TOP OF SAID REACTION RESSEL, SEPARATING FROMSAID WITHDRAWN MIXTURE A RECYCLE GAS CONSISTING LARGELY OF HYDROGEN,HEATING SAID RECYCLE GAS TO A TEMPEATURE HIGHER THAN THE AVERAGEREACTION TEMPERATURE, INTRODUCING THE LARGER PART OF THE HEATED RECYCLEGAS INTO THE SAID REACTION VESSEL NEAR THE BOTTOM THEREOF BELOW THEPOINT OF INTRODUCTION OF SAID FEED WHEREBY THE CATALYST IN THE LOWER